This change models existential Any and AnyObject using ExistentialType. Previously, these two existential types still were modeled with ProtocolCompositionType because they're not required to be written with any, but this allowed using any on these types in conformance constraint context, e.g.

func generic<T>(value: T) where T: any Any { ... } // okay??

This also forced all code that operates on existential types to handle both ExistentialType and ProtocolCompositionType. This change enables cleaning up all of that code, getting rid of TypeBase::isExistentialType in favor of TypeBase::is<ExistentialType>, adding ASTVerifier checks to ensure that no expressions have a protocol composition type to catch more bugs, etc

Note that Any and AnyObject still do not print with any in the ASTPrinter, except when it's needed to distinguish between the existential metatype and the singleton metatype, e.g. any Any.Type vs (any Any).Type.